FRESHWATER AND MARINE ORC.ANISMS. 333 



spite of their high concentration by freshwater Gammarns; there 

 is no interference with the regulatory activity of the integument. 

 At this point we approach the problem of the conditions for the 

 survival of the internal tissues, which are evidently very different 

 from the conditions which the external medium may impose 

 when an integument is interposed. 



IV. 



It .i[)p( -.irs, therefore, that freshwater or^ani-ms are strongly 

 c)i]tra-'ed to marine organisms with n-~pet t t> their ability to 

 adjust to change- in their chemical environment. Marine 

 organisms in general maintain a higher degree of interchange of 

 inorganic mati-ri.il> with their surroundings. 1 rc-hvvater organ- 

 isms, on tin- other hand, have a mon- restricted and selective 

 inten han^c. ami thus dissolved material- are kept in-ide their 

 bodii-- and water i- kept outside. In protecting them -el \ cs from 

 their en\ iri'iiment through the retention <>t -alts, freshwater 

 orgaiii'in- ha\e laid themselves open, paradoxically, to o-motic 

 di-turbance- of the integument. Marine <>rgani-m-, by allowing 

 the en\ ironing medium to serve as the phy-i< .logical fluid, are 

 able to endure a mn< h greater change in the en\ ironing medium, 

 than are hc-huater organisms with their greater regulation of 

 tlie internal medium. This greater chanv< is <>nr of concentia- 

 tion onlv . ho\\e\er, for both groups of animals are equally 

 susceptible to variations in the proportion- oi -alt- in the medium. 

 Thi- condition in the freshwater animal- i- entirely due to the 

 peculiar and \ariable type of permeability found in the intern 

 inent- of tin- ^ronp. \\hereby the\ r nonnalU" maintain a con- 

 ceiitratioii dilteieiKe between outside and in-ide fluid-, as ha- 

 been pointed out pre\ ioiisly (Quinton, '04, Adolph and Adolph. 

 '25). Thi- activity of the integument serves excelK-ntly in the 

 iiMiinal medium, but adds a factor of susceptibility. 



I xistence in the tn -huater medium is accompanied by the 

 ]>..-- --ion by all fn--huater organisms of relati\'ely dilute ti-Mie 

 fluids. Apparently it i- the integumentar\ activity which i> 

 upset by increased concentrations of the medium. Its up 

 in turn, produces rapid changes of a deleterious nature in the 

 composition of the body fluids. Marine organi>m>, on the other 

 hand, have body fluids which are in complete chemical c<|iiilib- 



